<script>
    var age = 38;
    var obj = {
        age: 18,
        getAge: function() {
            console.log(this.age);
        }
    };
    // this指向了谁? obj调用了getAge,所以this指向了obj
    obj.getAge();


    var fn = obj.getAge;
    // this指向了谁? this指向了window
    window.fn();
</script>